[Intl_DxMedPhys] Gamma Camera Max Count Rate Perpendicular versus Off-Angle

Wed Jun 25 14:57:34 EDT 2025

Not sure if this happens to everyone, but my listserv responses come back as an annoying mish-mash of separate email threads. Rather than replying to each one, I’m just going to say here: thank you all for the responses.

I do agree that it’s more important to use the same geometry if comparing data year-over-year. Plus, having the source perpendicular to the detector is obviously more clinical that having it at an awkward angle. My gut assumption would be that the perpendicular geometry would ‘optimize’ signal collection by having relatively even distribution of signals to all PMTs. Thus it seemed odd that you could somehow get a higher value by coming in off-axis. But the ideas presented have given me stuff to think about.

Evaluating count rate with the decay method would not be practical for me, but if I can get my hands on, or devise, the box-with-copper-plates test tool, then I will give that a try.

Dylan DeAngelis, MS, DABR
NYS Licensed Medical Physicist
(Diagnostic Radiological Physics and Medical Nuclear Physics)

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From: Bill Erwin <ervster57 at gmail.com>
Sent: Wednesday, June 25, 2025 11:56 AM
To: Martin, Michael <michael.martin at tamu.edu>
Cc: Dylan DeAngelis <ddeangelis at upstatemp.com>; Nima Kasraie <Nima.Kasraie at utsouthwestern.edu>; intl_dxmedphys_wd_osu_list at lists.osu.edu
Subject: Re: [Intl_DxMedPhys] Gamma Camera Max Count Rate Perpendicular versus Off-Angle

P. S. The follow-up question would be which UFOV dimension should be used in calculating the 5 UFOV distance ...

On Wed, Jun 25, 2025 at 9:18 AM Bill Erwin <ervster57 at gmail.com<mailto:ervster57 at gmail.com>> wrote:
I think you are on the correct track, Michael.  While performing that test intrinsically by moving a source inward from a starting distance is quite convenient (i.e., fast), the source radiation pattern transitions from closer to parallel (essentially uniform flux across the detector UFOV) to spherical (non-uniform flux weighted to that part of the detector closest to the source at a given distance).  The degree of non-uniformity in the flux, and how much of the UFOV begins to experience paralysis due to the Anger logic (some modern cameras have "fancy-schmancy" pulse pileup correction, multiple pulse integrators or other digital corrections), may depend on not only distance but the angle at which the source approaches the detector.

This may be why NEMA specifies the decay and copper plates methods with the source at a fixed and at the "magic" 5 UFOV or greater distance (although they allow the manufacturer to report results at a closer distance, but noting clearly that is the case).

"A camera under test shall have the camera crystal masked to the UFOV. The source shall be placed within a source holder, as in Figure 2-5, and shall be arranged as in Figure 2-1, except that the distance from the source to the detector surface may be less than five times the UFOV. The manufacturer shall report the distance that will produce the reported peak count rate value (note that the detector may not be fully irradiated at this distance). The source holder with the source shall be placed in front of the detector, so that the collimated cone of radiation is centered within the UFOV."

(It might be interesting to find out what happens with a multi-element, a.k.a. pixelated, detector.)

A good question for students or physics residents is why 5 UFOV is a "magic" (i.e., acceptable) distance, since ideal parallel flux is only obtained at infinite distance.

On Tue, Jun 24, 2025 at 11:38 AM Martin, Michael via Intl_dxmedphys_wd_osu_list <intl_dxmedphys_wd_osu_list at lists.osu.edu<mailto:intl_dxmedphys_wd_osu_list at lists.osu.edu>> wrote:
A guess(?): When the source is perpendicular/centered to the face of the camera, some number of PMTs directly in front of the source will be paralyzed, while those on the periphery will still be counting. When the source is at an angle, a fewer
A guess(?):

When the source is perpendicular/centered to the face of the camera, some number of PMTs directly in front of the source will be paralyzed, while those on the periphery will still be counting.

When the source is at an angle, a fewer number of PMTs near the edge with the source are paralyzed, while a larger number are still counting, due to the 1/r^2.

I’ve normally done ~ 1 mCi and roughly get the same distance (a few feet).

Best,

Michael

T. Michael Martin PhD, DABSNM, DABHP, LMP  |  Sr. Health Physicist and Assistant RSO
Environmental Health & Safety | Texas A&M University
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From: Intl_dxmedphys_wd_osu_list <intl_dxmedphys_wd_osu_list-bounces+michael.martin=tamu.edu at lists.osu.edu<mailto:tamu.edu at lists.osu.edu>> On Behalf Of Dylan DeAngelis via Intl_dxmedphys_wd_osu_list
Sent: Tuesday, June 24, 2025 11:16 AM
To: Nima Kasraie <Nima.Kasraie at UTSouthwestern.edu<mailto:Nima.Kasraie at UTSouthwestern.edu>>; 'intl_dxmedphys_wd_osu_list at lists.osu.edu<mailto:intl_dxmedphys_wd_osu_list at lists.osu.edu>' <intl_dxmedphys_wd_osu_list at lists.osu.edu<mailto:intl_dxmedphys_wd_osu_list at lists.osu.edu>>
Subject: Re: [Intl_DxMedPhys] Gamma Camera Max Count Rate Perpendicular versus Off-Angle

Hi Nima, What do you consider ‘too close’? The quickest reference I could find is TG 177, which says to use 1 mCi, but doesn’t say about how close that should get you to the detector surface. We aim for between 0. 5 and 1 mCi so that we can
Hi Nima, What do you consider ‘too close’? The quickest reference I could find is TG 177, which says to use 1 mCi, but doesn’t say about how close that should get you to the detector surface. We aim for between 0. 5 and 1 mCi so that we can use
Hi Nima,

What do you consider ‘too close’? The quickest reference I could find is TG 177, which says to use 1 mCi, but doesn’t say about how close that should get you to the detector surface. We aim for between 0.5 and 1 mCi so that we can use the same point source for sensitivity testing. I’d say we usually hit the max count rate around 2-3 feet from the detector.

Dylan DeAngelis, MS, DABR
NYS Licensed Medical Physicist
(Diagnostic Radiological Physics and Medical Nuclear Physics)

Upstate Medical Physics – Diagnostic Radiology, Medical Nuclear & Medical Health Physics, P.C.
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From: Intl_dxmedphys_wd_osu_list <intl_dxmedphys_wd_osu_list-bounces at lists.osu.edu<mailto:intl_dxmedphys_wd_osu_list-bounces at lists.osu.edu>> On Behalf Of Nima Kasraie via Intl_dxmedphys_wd_osu_list
Sent: Tuesday, June 24, 2025 11:14 AM
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Subject: Re: [Intl_DxMedPhys] Gamma Camera Max Count Rate Perpendicular versus Off-Angle

I’ve also noticed angular dependencies. But I usually ignore, since I treat this as more of a consistency check. For my Symbia Intevo Bolds, I’ve set the pass/fail criteria at +/- 5% of Siemens reference value: One also notices variations depending
I’ve also noticed angular dependencies. But I usually ignore, since I treat this as more of a consistency check. For my Symbia Intevo Bolds, I’ve set the pass/fail criteria at +/- 5% of Siemens reference value:

[cid:image001.png at 01DBE5E0.FA2C3DC0]

One also notices variations depending on what window you use as well.

Plus, are you maybe too close to the head? I use 5mCi.

Nima

[cid:image002.png at 01DBE5E0.FA2C3DC0]
Nima Kasraie, PhD, MSc, DABR, DABSNM
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From: Intl_dxmedphys_wd_osu_list <intl_dxmedphys_wd_osu_list-bounces+nima.kasraie=utsouthwestern.edu at lists.osu.edu<mailto:intl_dxmedphys_wd_osu_list-bounces+nima.kasraie=utsouthwestern.edu at lists.osu.edu>> On Behalf Of Dylan DeAngelis via Intl_dxmedphys_wd_osu_list
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Subject: [Intl_DxMedPhys] Gamma Camera Max Count Rate Perpendicular versus Off-Angle

Good morning, We assess max count rate on a gamma camera by moving a point source (usually 0.  5-1 mCi) closer to the detector until we observe the peak rate, before it starts to fall off again due to the paralyzable dead time. I’ve

Good morning, We assess max count rate on a gamma camera by moving a point source (usually 0. 5-1 mCi) closer to the detector until we observe the peak rate, before it starts to fall off again due to the paralyzable dead time. I’ve always
Good morning,

We assess max count rate on a gamma camera by moving a point source (usually 0.5-1 mCi) closer to the detector until we observe the peak rate, before it starts to fall off again due to the paralyzable dead time. I’ve always done this with the activity perpendicular to the surface of the detector. But we recently noticed that if you approach the detector at an angle, you can reach a higher count rate. I don’t know if this is true for every unit – I noticed it first on a Siemens Symbia and then made the same observation on an old Philips Vertex. The change is pretty substantial. On the Vertex, measuring perpendicular I observed around 205 kcps, but from an angle I managed to get 236 kcps.

In both cases, the dead time of the counting system should be fixed, so there should only be one max count rate, right? How could I be getting two different values based on the angle of approach? Has anyone else seen this?

Dylan DeAngelis, MS, DABR
NYS Licensed Medical Physicist
(Diagnostic Radiological Physics and Medical Nuclear Physics)

Upstate Medical Physics – Diagnostic Radiology, Medical Nuclear & Medical Health Physics, P.C.
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